The implementation of digital networks, more precisely Private Circuit Switching Networks such as Integrated Service Digital Networks (ISDN), began in the early sixties with the process of digitization of telephone networks in different countries replacing the earlier analogue technologies.
CCITT Recommendations provide principles and guidelines on the ISDN concept and ISDN standards, and also detailed specifications of the user-network and inter-network interfaces.
CCITT Recommendation I. 430 defines layer-1 characteristics of the user-network interface to be applied at the S or T reference points for the basic interface structure. ISDN basic access provides a 144 kb/s transmission channel, which is divided into three distinct subchannels by means of a Time Division Multiplex mechanism: two so-called B channels and a single D channel. Each of the B channels is an independent 64 kbps duplex bearer channel with bit and octet timing, which can be used for a digital telephone communication (the Pulse Code Modulation [PCM] is performed by the subscriber equipment) or for a 64 kbps data communication. The D channel is a 16 kbps channel which allows dynamic multiplexing of the following information: signalling information relative to the B-channels, information relative to low rate teleservices, and information using low rate Packet Switching services (from 16 to 9600 bps).
Although a 64 kbps B channel is sufficient for a wide variety of data communication applications, particularly those using conventional data terminating equipment interfaces such as described in CCITT recommendations V24, V35, and X21, a higher transmission rate through one ISDN network may appear desirable. For instance, in multimedia applications where data, voice and images are simultaneously transmitted, the user may wish to have a 128 kbps channel.
Known ISDN equipment does not allow the use of both independent B channels in order to provide a full 128 kbps data communication channel, since both B channels operate independently and asynchronously. Indeed, although the position of one bit in a byte is maintained through the ISDN transmission network, the transmission time of that byte in the network can largely vary from one B channel to the other, thus preventing their synchronization.